1. Field of the Invention
The present invention relates to a method and an apparatus for continuously producing foamed metals, and more particularly to a method and an apparatus for continuously producing an ultralight foamed aluminum or aluminum alloy by conveying molten metal furnished from a viscosity-enhancing furnace to a foaming furnace, and injecting gas into the conveyed foaming furnace while agitating the molten metal using an electronic agitating system.
2. Description of the Related Art
Foamed metals, in particular foamed aluminums contain a number of foams in a metal matrix. The foamed metals are similar to polyurethane, foamed glasses or foamed concrete in appearance.
Since foamed aluminum commonly has a porosity of above 90%, its specific gravity is within the range of from 0.2 to 1.0 and thus it is very light. Accordingly, foamed aluminum is an excellent material for sound absorption, sound insulation and shock absorption. In addition, since the matrix is aluminum, the foamed aluminum has excellent fire resistance, heat resistance, strength, and workability. Due to these properties, the foamed aluminum is also used as a functional material. Furthermore, the foamed aluminum has decorative surface peculiar to aluminum, and thus can be used as an interior finish for construction. To these uses, it is expected that the foamed aluminum can be used as a sound absorption material and is useful as a material in the manufacture of automobiles, vehicles, ships, etc., using the lightness and strength of foamed aluminum.
For better properties, foamed aluminum must have an uniformity in type and size of foams and must be produced in a simpler process.
U.S. Pat. No. 5,112,697 discloses a process for continuously producing stabilized metal foam body by dispersing stabilized particles such as Al2O3 in a metal matrix, infiltrating molten metal into the stabilized particles, and injecting gas into a space inside an impeller while rotating the impeller. However, this process has limitations that the stabilized particles must impart a uniform viscosity to the molten metal and prevent them from being drained. Accordingly, the volume fraction and size of stabilized particles are very important factors. In addition, the shape and angle of impeller are important factors since gas injection is accomplished through hollows of impeller. For these reasons, control system must be sophisticated. Otherwise, foamed metals with low quality may be produced. Accordingly, there remains a need to develop a method for continuously producing foamed metals with high quality in a simpler manner.
The conventional method for continuously producing foamed metals increases the viscosity of the foamed metals and trap the generated foams by using stabilized particles. It maintains sufficient viscosity by adding Ca—Al alloy to a separate melting furnace before foams occur in molten metal. The viscosity is also increased by controlling a solid state percentage by adjusting the temperature of molten metal below liquefying temperature. Gas injection has made by placing a membrane on the bottom of furnace and injecting a gas into molten metal through the membrane. Then, the injected gas is homogeneously distributed in the molten metal by rotating an impeller arranged in the furnace. And thereafter a foamed metal is obtained by coagulating the molten metal. This method has many advantages that expensive stabilized particles have no need to be added, and processes such as selection of stabilized particles and preheat can be omitted. In addition, processing time is minimized since the viscosity of molten metal is previously adjusted. The gas injection through membrane is safer, and the used impeller is manufactured by a simpler manner, compared with conventional impeller.
However, since all these methods always use impellers. It is difficult to select a material suitable for each impeller. Further, breakage of the impeller influences the quality of the final product, and generates a troublesome such as impeller exchange. Agitating blades cannot smoothly agitate molten metal because of the increased viscosity of the molten metal due to addition of a viscosity-enhancing agent.
Therefore, a method for continuously producing foamed metals with high quality must satisfy the requirements as below:
i) molten metal has a uniform viscosity so as to trap foams in the molten metal. This inhibits foams contained in the molten metal from running out from the molten metal; ii) foams trapped in the molten metal must be homogenously dispersed within the molten metal. For this purpose, the efficiency of agitator must be improved; and iii) impeller must not be broken during agitating, and the agitation must be softly carried out.